Garbage collection is a widely used technique that frees the programmer from
having to know the life-times of heap objects, making software easier to produce
and maintain. Many programming languages rely on garbage collection for
automatic memory management. There are two primary forms of garbage collection:
conservative and accurate.

Conservative garbage collection often does not require any special support
from either the language or the compiler: it can handle non-type-safe
programming languages (such as C/C++) and does not require any special
information from the compiler. The [LINK] Boehm collector is an example of a
state-of-the-art conservative collector.

Accurate garbage collection requires the ability to identify all pointers in
the program at run-time (which requires that the source-language be type-safe in
most cases). Identifying pointers at run-time requires compiler support to
locate all places that hold live pointer variables at run-time, including the
processor stack and registers.

Conservative garbage collection is attractive because it does not require any
special compiler support, but it does have problems. In particular, because the
conservative garbage collector cannot know that a particular word in the
machine is a pointer, it cannot move live objects in the heap (preventing the
use of compacting and generational GC algorithms) and it can occasionally suffer
from memory leaks due to integer values that happen to point to objects in the
program. In addition, some aggressive compiler transformations can break
conservative garbage collectors (though these seem rare in practice).

Accurate garbage collectors do not suffer from any of these problems, but they
can suffer from degraded scalar optimization of the program. In particular,
because the runtime must be able to identify and update all pointers active in
the program, some optimizations are less effective. In practice, however, the
locality and performance benefits of using aggressive garbage allocation
techniques dominates any low-level losses.

This document describes the mechanisms and interfaces provided by LLVM to
support accurate garbage collection.

LLVM provides support for a broad class of garbage collection algorithms,
including compacting semi-space collectors, mark-sweep collectors, generational
collectors, and even reference counting implementations. It includes support
for read and write barriers, and associating meta-data with stack objects (used for tagless garbage
collection). All LLVM code generators support garbage collection, including the
C backend.

We hope that the primitive support built into LLVM is sufficient to support a
broad class of garbage collected languages, including Scheme, ML, scripting
languages, Java, C#, etc. That said, the implemented garbage collectors may
need to be extended to support language-specific features such as finalization,
weak references, or other features. As these needs are identified and
implemented, they should be added to this specification.

LLVM does not currently support garbage collection of multi-threaded programs or
GC-safe points other than function calls, but these will be added in the future
as there is interest.

This section describes the interfaces provided by LLVM and by the garbage
collector run-time that should be used by user programs. As such, this is the
interface that front-end authors should generate code for.

The llvm.gcroot intrinsic is used to inform LLVM of a pointer variable
on the stack. The first argument contains the address of the variable on the
stack, and the second contains a pointer to metadata that should be associated
with the pointer (which must be a constant or global value address). At
runtime, the llvm.gcroot intrinsic stores a null pointer into the
specified location to initialize the pointer.

Consider the following fragment of Java code:

{
Object X; // A null-initialized reference to an object
...
}

This block (which may be located in the middle of a function or in a loop nest),
could be compiled to this LLVM code:

Entry:
;; In the entry block for the function, allocate the
;; stack space for X, which is an LLVM pointer.
%X = alloca %Object*
...
;; "CodeBlock" is the block corresponding to the start
;; of the scope above.
CodeBlock:
;; Initialize the object, telling LLVM that it is now live.
;; Java has type-tags on objects, so it doesn't need any
;; metadata.
call void %llvm.gcroot(%Object** %X, sbyte* null)
...
;; As the pointer goes out of scope, store a null value into
;; it, to indicate that the value is no longer live.
store %Object* null, %Object** %X
...

Several of the more interesting garbage collectors (e.g., generational
collectors) need to be informed when the mutator (the program that needs garbage
collection) reads or writes object references into the heap. In the case of a
generational collector, it needs to keep track of which "old" generation objects
have references stored into them. The amount of code that typically needs to be
executed is usually quite small (and not on the critical path of any
computation), so the overall performance impact of the inserted code is
tolerable.

To support garbage collectors that use read or write barriers, LLVM provides
the llvm.gcread and llvm.gcwrite intrinsics. The first
intrinsic has exactly the same semantics as a non-volatile LLVM load and the
second has the same semantics as a non-volatile LLVM store, with the
additions that they also take a pointer to the start of the memory
object as an argument. At code generation
time, these intrinsics are replaced with calls into the garbage collector
(llvm_gc_read and llvm_gc_write respectively), which are then
inlined into the code.

If you are writing a front-end for a garbage collected language, every load or
store of a reference from or to the heap should use these intrinsics instead of
normal LLVM loads/stores.

The llvm_gc_initialize function should be called once before any other
garbage collection functions are called. This gives the garbage collector the
chance to initialize itself and allocate the heap spaces. The initial heap size
to allocate should be specified as an argument.

The llvm_gc_collect function is exported by the garbage collector
implementations to provide a full collection, even when the heap is not
exhausted. This can be used by end-user code as a hint, and may be ignored by
the garbage collector.

Implementing a garbage collector for LLVM is fairly straight-forward. The LLVM
garbage collectors are provided in a form that makes them easy to link into the
language-specific runtime that a language front-end would use. They require
functionality from the language-specific runtime to get information about where pointers are located in heap objects.

The llvm_cg_walk_gcroots function is a function provided by the code
generator that iterates through all of the GC roots on the stack, calling the
specified function pointer with each record. For each GC root, the address of
the pointer and the meta-data (from the llvm.gcroot intrinsic) are provided.

The three most common ways to keep track of where pointers live in heap objects
are (listed in order of space overhead required):

In languages with polymorphic objects, pointers from an object header are
usually used to identify the GC pointers in the heap object. This is common for
object-oriented languages like Self, Smalltalk, Java, or C#.

If heap objects are not polymorphic, often the "shape" of the heap can be
determined from the roots of the heap or from some other meta-data [Appel89, Goldberg91, Tolmach94]. In this case, the garbage collector can
propagate the information around from meta data stored with the roots. This
often eliminates the need to have a header on objects in the heap. This is
common in the ML family.

If all heap objects have pointers in the same locations, or pointers can be
distinguished just by looking at them (e.g., the low order bit is clear), no
book-keeping is needed at all. This is common for Lisp-like languages.

The LLVM garbage collectors are capable of supporting all of these styles of
language, including ones that mix various implementations. To do this, it
allows the source-language to associate meta-data with the stack roots, and the heap tracing routines can propagate the
information. In addition, LLVM allows the front-end to extract GC information
from in any form from a specific object pointer (this supports situations #1 and
#3).

To make this more concrete, the currently implemented LLVM garbage collectors
all live in the llvm/runtime/GC/* directories in the LLVM source-base.
If you are interested in implementing an algorithm, there are many interesting
possibilities (mark/sweep, a generational collector, a reference counting
collector, etc), or you could choose to improve one of the existing algorithms.

SemiSpace is a very simple copying collector. When it starts up, it allocates
two blocks of memory for the heap. It uses a simple bump-pointer allocator to
allocate memory from the first block until it runs out of space. When it runs
out of space, it traces through all of the roots of the program, copying blocks
to the other half of the memory space.

Possible Improvements

If a collection cycle happens and the heap is not compacted very much (say less
than 25% of the allocated memory was freed), the memory regions should be
doubled in size.